This application relates to wind turbine generators (WTGs).
Wind energy has emerged as the fastest growing source of energy, offering a clean, renewable, and ecological-friendly alternative to fossil-based energy supplies. At the present growth rate, wind energy conversion is projected to produce more than 117,000 MW by the year of 2009, claiming about 1.25% of the global electricity generation. In addition to their traditional role in servicing rural residences in grid-isolated areas, wind turbine generators are now increasingly installed in large-scale (e.g., multi-megawatt) wind farms and integrated into power grids that can deliver electricity to consumers nationwide.
The performance of a grid-connected WTG can be influenced by many factors, such as voltage fluctuations on the grid. For example, a short circuit on the grid may result in a sudden voltage drop, which reduces the effective drag on the WTG and may cause both the turbine and the generator to accelerate rapidly. To ensure safe operation, some WTGs have been designed to trip off-line (i.e., disconnect from the grid and shut down) as soon as grid voltage drops below a prescribed level (e.g., 85% of nominal voltage). After fault clearance, these WTGs enter a restart cycle that can last several minutes before resuming power transmission to the grid.
During this off-line period, the loss of power generation may impact the stability of utility grids to which WTGs are connected. As the number of grid-integrated wind plants/farms continues to grow, regulatory agencies in many countries have started to adopt strict interconnection standards that require large WTGs to remain online during disturbances and continue to operate for an extended period—a process called “low-voltage ride through” (LVRT).
Among various interconnection standards, the Spanish Grid Code, for example, requires WTGs to be able to sustain (“ride-through”) line voltage at 20% of rated level for at least 500 ms. FIG. 1A shows an example of voltage transients when a low-voltage event occurs. In this case, after an initial dip of 500 ms, line voltage starts to recover and within 15 seconds has returned to 95% of nominal. During the entire low-voltage period (˜15 s), the Spanish Grid Code requires WTGs to continue to operate and supply current in controlled amounts to help stabilize the grid. FIG. 1B shows the required current behavior, measured by the ratio of the magnitude of reactive current to total current (Ireactive/Itotal) as a function of line voltage. Note that other countries may have different regulations on grid-connected WTGs' current and voltage behaviors in response to low voltage disturbances.